Am 08.10.2011 16:04, schrieb Captain Freako:
Hi all,
In this definition from the Parsec library:
parse :: (Stream s Identity t) => Parsec s () a -> SourceName -> s -> Either ParseError a parse p = runP p ()
what's the significance of `Identity t'? (`t' isn't used anywhere.)
http://hackage.haskell.org/packages/archive/parsec/3.1.2/doc/html/Text-Parse... Text.Parsec.Prim contains class Monad m => Stream s m t | s -> t where saying that the type s determines t (that is a functional dependency). The instances are: Monad m => Stream ByteString m Char Monad m => Stream ByteString m Char Monad m => Stream Text m Char Monad m => Stream Text m Char Monad m => Stream [tok] m tok so usually you have a character stream. (There are lazy and strict version of Text and ByteString.) The last instance also works for plain strings (String = [Char]). Using "Identity" for the monad m just means, that you actually do not need a monad (but need to supply a dummy one). If you want to keep thinks simpler you could use packages parsec1 or parsec2 Cheers Christian http://hackage.haskell.org/package/parsec2 http://hackage.haskell.org/package/parsec1
Thanks, -db